As well known, the primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide and unburned hydrocarbons. Also well known is the fact that oxidation of molecular nitrogen in air-breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. As temperature rises, for example, in the combustor, the rate of chemical reactions forming oxides of nitrogen increase exponentially. However, if the temperature of the combustion chamber hot gas is controlled to a lower level, thermal NO.sub.x will be produced at very low rates.
One method of controlling the temperature of the reaction zone of a combustor at levels at which minimal thermal NO.sub.x is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premix combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where minimal NO.sub.x is formed. One problem associated with premix combustion is that the fuel/air mixture strength must be reduced to a level close to the lean flammability limit for most hydrocarbon fuels. As a consequence, lean premixed combustors tend to be less stable than more conventional diffusion flame combustors and do not provide adequate turndown for operation over the entire load range of the turbine. It is highly desirable to obtain the best possible emissions performance over the entire gas turbine operating range from ignition through mid-load while burning a diffusion flame, and mid-load to full load while burning a premix flame.
Burners with diffusion and premix capability for heavy duty industrial gas turbines are known. For example, in prior combustors of that type, all of the air brought into the premix chamber is used for both diffusion and premix combustion modes. Thus, while the air supply may be optimal for premixed combustion mode, the injection of fuel for the diffusion combustion mode into the same total air supplied the premix chamber, simply made the diffusion flame performance non-optimal, e.g., lack of stability of the flame. Other prior combustors employ two separate passages for supplying air in premix and diffusion combustion modes. Where swirlers have been used, to applicant's knowledge, they have not been swirlers having aerodynamic vanes but, rather, flat vanes which cannot be used for flowing air through the air passage for diffusion and premix combustion modes. Thus, two very separate and distinct passages were previously used for premix and diffusion combustion modes and, accordingly, a richer fuel/air ratio in a premix mode and higher NO.sub.x resulted. Further, older combustors employed two distinct air inlets at axially spaced positions along the combustor to achieve diffusion and premix combustion modes.